Produced crude oil in the field can have substantial quantities of water associated with it. The water-cut or amount of water associated with the oil can be as high as 95% of the total produced stream. This is especially true in heavy oil fields where the oil is being produced from reservoir(s) having a strong water drive. Usually, the heavy oil itself is so viscous at ambient temperatures that it requires tremendous pumping energy to make it flow, if at all. The water present in the produced stream can be classified into two categories: "bound" water and "free" water. "Bound" water is that water which is locked up in the oil as a water-in-oil (W/O) emulsion. Separating this water from the stream typically requires applying the appropriate combination of heat, mixing and a chemical demulsifier. "Free" water is that water which is relatively loosely held up by the oil and can be removed just by heating the stream to the right temperature.
The above-mentioned produced water-in-oil (W/O) emulsions usually have a higher viscosity than the dry oil which itself is very viscous. This high viscosity frequently limits the rate at which the W/O emulsion, and hence the oil contained in it, can be pumped up a wellbore or through a pipeline. One method for handling this problem has been to formulate an oil-in-water (O/W) emulsion of the oil. Oil-in-water emulsions usually have a lower viscosity than the oil itself and so the oil in this form can be pumped at faster rates. Crude oil-in-water emulsions have been formulated in one of two ways:
One approach has been to take the produced stream from the wellbore and separate out the water by subjecting it to a combination of heat, mixing and at least one chemical demulsifier in a heater-treater. The "dry" oil stream which may contain anywhere from 1-5% water by weight is then mixed with the right amount of water and a chemical emulsifying agent to form a low viscosity, transportable oil-in-water emulsion. The amount of water used is governed by the need to obtain a low viscosity transport fluid and to maximize the oil throughput. Normally, a transport O/W emulsion contains from about 15% to about 35% water by weight.
The other approach has been to attempt to form an oil-in-water emulsion within the wellbore itself. Water containing one or more emulsifying agent(s) is usually added either down the annulus or the tubing to contact the oil and water coming from the formation into the wellbore before or as they enter the downhole pump. In this way, an O/W emulsion of the crude is formed as the fluids pass through the downhole pump. This downhole attempt at forming O/W emulsions presents considerable operational difficulties. Each well behaves independently of any other well. There are presented, therefore, a number of operational variables from well to well which must be constantly combatted if a suitable O/W emulsion is to be formed. More serious is that, in order to produce the oil to the surface, it is necessary to use some artificial lifting device, and where water content is high, energy requirements for the lifting devices are also high. This will affect the chemical dosage used. For example, in the case of heavy oil wells with high water cuts wherein enormous amounts of total fluid (oil plus water) have to be lifted to get reasonable oil production rates, it is becoming common to use electrical submersible pumps (ESP) which can pump out these fluids at tremendous rates. The formation of an O/W emulsion is determined by the temperature, chemical emulsifier dosage and degree of shear or mixing. In a well using an ESP which generates a lot of shear, an excessive amount of chemical may be required to successfully formulate, if at all, an O/W emulsion.